On 25.07.2012 12:44, Avi Kivity wrote: > On 07/24/2012 06:52 PM, Cong Wang wrote: > >>> From 6b679d1af20656929c0e829f29eed60b0a86a74f Mon Sep 17 00:00:00 2001 >>> From: Stefan Bader >>> Date: Fri, 13 Jul 2012 15:16:33 +0200 >>> Subject: [PATCH] x86/mm: Limit 2/4M size calculation to x86_32 >>> >>> commit 722bc6b (x86/mm: Fix the size calculation of mapping tables) >>> did modify the extra space calculation for mapping tables in order >>> to make up for the first 2/4M memory range using 4K pages. >>> However this setup is only used when compiling for 32bit. On 64bit >>> there is only the trailing area of 4K pages (which is already added). >>> >>> The code was already adapted once for things went wrong on a 8TB >>> machine (bd2753b x86/mm: Only add extra pages count for the first memory >>> range during pre-allocation early page table space), but it looks a bit >>> like it currently would overdo things for 64bit. >>> I only noticed while bisecting for the reason I could not make a crash >>> kernel boot (which ended up on this patch). >>> >>> Signed-off-by: Stefan Bader >>> Cc: WANG Cong >>> Cc: Yinghai Lu >>> Cc: Tejun Heo >> >> Acked-by: Cong Wang >> >> Sorry for that I was not aware of x86_64 is different with x86 in the >> first 2/4M. > > Why would there be a difference? > > Shouldn't the IO space at 0xa0000-0x100000 be mapped with uncacheable > attributes (or WC for VGA)? If it's done later, it can be done later > for both. > arch/x86/mm/init.c unsigned long __init_refok init_memory_mapping(... ... ifdef CONFIG_X86_32 /* * Don't use a large page for the first 2/4MB of memory * because there are often fixed size MTRRs in there * and overlapping MTRRs into large pages can cause * slowdowns. */